The desktop Pentium 4 processor is not SMP capable.
None of the desktop Pentium 4 chipsets support 64-bit PCI slots either: the SiS645/645DX, the i845 family and i850 family do not have 64-bit slot support.
Strictly speaking, the external P64H hub can be added to to the i845 and i850 chipsets, although no manufacturer does this.
If you wanted dual Pentium 4s with 64-bit PCI slots, you will need to go with an i860 board for XEON processors. These are probably outside of your price range.